A key study of the effectiveness of an Exact Sciences test aimed at early detection of the disease nears completion. If it works, the company's shares should get healthier.

Colon cancer has been called the most preventable, but least prevented, form of cancer. There are more than 143,000 new cases and about 50,000 deaths from the disease in the U.S. each year, making it second only to lung cancer in annual cancer mortality.

If people over 50 adhere to medical-screening guidelines and get colonoscopies every 10 years, the odds of dying from colon cancer are very low because doctors can see and remove precancerous growths during the exam. It can take a decade or more for these precancerous...